Downlink carrier aggregation is a bandwidth expansion method used to increase the data rate for a user by allowing the user to receive data simultaneously on multiple carriers. High throughput can be obtained by utilizing adaptive coding and modulation while taking into account reported Channel State Information (CSI) reports into the scheduling operation, when combined with Hybrid Automatic Repeat reQuest (HARQ) feedback signaling in the uplink. The Uplink Control Information (UCI) signaling, e.g., HARQ Acknowledgement (HARQ-ACK), periodic CSI, aperiodic CSI, Scheduling Request (SR), constitutes uplink overhead which typically scales with the number of aggregated carriers. It is a non-trivial task to design the UCI feedback mechanisms, as reducing the amount of UCI information improves the reliability of the UCI and the coverage of the uplink (i.e., lower coding rates can be used) but may on the other hand adversely result in worse downlink throughput performance, i.e., if less HARQ-ACKs or CSI reports become available at base station (e.g., the E-UTRAN NodeB or evolved NodeB (eNodeB)).
As more frequency bands become available for the operators (e.g., using unlicensed spectrum also for cellular systems) and with a continued development of the device capabilities, the number of aggregated downlink carriers is expected to significantly increase in the future. On the other hand, it is considerably more expensive and complicated to produce devices capable of uplink carrier aggregation. Hence, a typical case is where the device can receive on multiple downlink carriers but is only able to transmit the UCI feedback on a single uplink carrier. Therefore, the UCI feedback mechanism may become a bottleneck if it requires transmission on a large number of uplink time-frequency resources. It is thus desirable to avoid feeding back UCI which provides no meaningful information to the eNodeB. One particular such example is HARQ-ACK information related to a carrier which is not scheduled, for which the device feeds back a Negative ACK (NACK), which may be disregarded at the base station since it knows on which carriers data was scheduled.
A further issue when increasing the number of aggregated downlink carriers is an increase in uplink intra-cell interference. Typically, the device is detecting a downlink control channel for each aggregated carrier, which provides information necessary for detecting the downlink data channel. An error case happens when the device detects a downlink control channel which is aimed for another device, or if it detects a downlink control channel although the eNodeB never transmitted any. These false detection events will result in that the device will try to detect the data channel and initiate a HARQ-ACK transmission and feed back a Negative ACK (NACK), since the associated data channel either was not transmitted or cannot be successfully detected. The device will therefore use uplink channel resources for UCI feedback which the eNodeB either does not expect to be used or it expects them to be used by another User Equipment (UE), thereby creating unexpected uplink intra-cell interference.
Therefore in order to accommodate systems with aggregation of large number of downlink carriers, it needs to be assured that feedback of unnecessary UCI information is reduced, while maintaining a high reliability of the decoded UCI at the eNodeB.
The 3GPP Long Term Evolution Advanced (LTE-Advanced) system is capable of downlink carrier aggregation between Frequency Division Duplex (FDD) carriers, between Time Division Duplex (TDD) carriers and between mixtures of FDD and TDD carriers. This system has supported carrier aggregation of up to 5 downlink carriers until Rel-12 and will be enhanced to support up to 32 downlink carriers in Rel-13 (1 Primary serving cell (PCell) and 31 Secondary serving cells (SCells)). Each transport block (TB) on the Physical Downlink Shared Channel (PDSCH) is associated with one HARQ-ACK bit, and at most 2 TBs can be transmitted on a carrier in a subframe. This could potentially yield 64 HARQ-ACK bits when the PCell is FDD and 618 bits when the PCell is TDD. The HARQ-ACK bits can be transmitted in the Physical Uplink Control Channel (PUCCH) or in the Physical Uplink Shared Channel (PUSCH). Different type of block codes and repetition codes are used for the HARQ-ACK, depending on the number of bits.
For FDD, each downlink subframe is associated with a corresponding uplink subframe used for transmitting the HARQ-ACK. For TDD, a set of downlink subframes is associated with an uplink subframe, wherein the HARQ-ACKs are transmitted. In each downlink subframe with a downlink assignment signaled on the downlink control channel (Physical Downlink Control Channel (PDCCH)/Enhanced PDCCH (EPDCCH)), the downlink control channel contains a two bit incremental counter, the Downlink Assignment Index (DAI), which is incremented for each subframe that has been scheduled among the set of downlink subframes. Furthermore, in TDD, uplink grants for the PUSCH signaled on the downlink control channel contain a two bit value, the uplink DAI, which indicates the total number of subframes which have been scheduled among the downlink subframes.
The number of HARQ-ACK feedback bits is semi-statically configured (by RRC signaling) and depends on the number of configured carriers, the transmission mode and, for TDD, the UL/DL configuration. A consequence is therefore that the UE will feedback a NACK if it was not scheduled in a given subframe, which will induce unnecessary overhead. In order to reduce the uplink overhead (i.e., the number of occupied resource elements), it was suggested to dynamically adjust the number of HARQ-ACK bits based on the actual number of scheduled carriers. This requires dynamic signaling from the eNodeB to the UE of the number of HARQ-ACK bits to be fed back. Dynamic signaling by means of physical control channels is faster but less reliable than RRC signaling. It is therefore an issue if the signaling is not detected correctly, causing the UE and the eNodeB to assume different number and/or order of the HARQ-ACK bits. The consequence maybe severe since that will generate significant amount of ACK-to-NACK and NACK-to-ACK errors of the HARQ-ACK bits.